Static exciter systems for feeding the field winding creating a magnetic excitation field for a generator and thus supplying the generator excitation are widely used. These systems are determined by their robustness and a short response time. In case of grid faults it is important that generators connected to the grid, especially synchronous generators mainly used for power generation, remain in synchronism. The ability of synchronous generators to keep a synchronous run in an electric grid system is referred to as transient stability in a power system. Power system transient stability is a complex problem which involves many phenomena. Sometimes local measurements at one generator do not capture the complexity of system wide problems and grid faults cannot be determined by this. Monitoring and controlling system wide problems beyond monitoring and controlling separate generators can be addressed by means of Wide Area Measurement Systems (WAMS) recently developed.
The patent EP 1 805 887 B1 describes an ultracapacitor excitation booster to improve the transient stability of synchronous generators in case of grid faults. Such an excitation booster system is described below in more detail with reference to FIG. 1. However, adding excitation boosters to the generators as well as other influences at the grid, and controlling the generators by terminal voltage may deteriorate the grid system stability. It is observed that generators accelerate when a grid fault occurs. Further, is has been observed that some generator rotors accelerate with respect to a comparative value whereas other generators decelerate. Among others applying excitation booster systems to the grid system and thereby influencing the field voltage of the generator rotor might thus lead to braking the rotor speed. The braking of some generator rotors in a system of connected generators increases the speed deviation with respect to the comparative value. These speed deviations between the rotor speeds of the connected generators impair the transient stability of the whole grid system.